1. Field of the Invention
This invention relates to a system and method for removing nitrogen and producing liquefied natural gas (“LNG”) from gaseous streams containing methane and other impurities without the need for an external refrigeration system. The invention also relates to a system and method for removing nitrogen from methane and for producing liquefied nitrogen in addition to LNG. The system and method of the invention are particularly suitable for use in recovering and processing comparatively small volumes of methane from coal mine vent streams or streams containing methane and nitrogen captured as flash gas at an LNG loading site.
2. Description of Related Art
Because many sources of methane produced during mining, energy transport or other industrial applications are not located near a natural gas transmission pipeline or other facility having gas-processing or liquefaction capabilities, a significant amount of methane gas, often combined with other gaseous or vaporous components, is either flared or vented to the atmosphere. This is particularly true in remote or otherwise underdeveloped areas where environmental impact is less of a concern than in the United States and other developed countries.
Naturally occurring methane is often encountered in coal mines, where it poses a significant risk to miners and to the mine subsurface equipment and inventory. This risk arises from miners being unable to breathe methane gas and also because air containing more than about 5 percent methane (preferably not more than about 2 percent) poses a significant risk of explosion. For these reasons, vertical shafts are frequently drilled into coal-containing formations ahead of the mining equipment so that any pockets of methane encountered during the drilling can be brought to the surface. Air is also forced down into subterranean mines and circulated through the mine shafts to dilute any residual methane that may be present and force it to the surface as well. Once the mining equipment reaches the vertical shafts drilled to recover methane from the formation, collapses can occur that produce another kind of methane-containing gas referred to as “gob gas,” which is also extremely hazardous.
Also, at LNG loading facilities, some LNG is typically vaporized as flash gas when the product first enters the tank, which is typically in an LNG tanker or other transport vessel. Because LNG normally comprises a minor amount of residual nitrogen, and because the nitrogen vaporizes at a lower temperature than LNG, the flash gas thus produced will contain a higher percentage of nitrogen than is contained in the LNG. For this reason, even where the flash gas is captured without exposing it to air, the methane in the flash gas cannot readily be re-liquefied without first removing the nitrogen. Although the amount of methane in the flash gas is relatively minor compared to the total amount being loaded, it may not enough to justify economically the investment and expense required to remove the nitrogen and then re-liquefy the methane using conventional technology. Unfortunately, this can cause operators to resort to the more expedient but less environmentally responsible alternatives of venting or flaring the flash gas.
Advantages of recovering coal mine methane for producing LNG, the existing technologies and the importance of accommodating smaller gas flows than conventional natural gas to LNG applications are all discussed in “Coal Mine Methane and LNG,” a paper published in November 2008 by the U.S. Environmental Protection Agency Coalbed Methane Outreach Program Technical Options Series.
Prior patents disclosing other gas processing technology invented by Rayburn C. Butts of BCCK Engineering include U.S. Pat. Nos. 5,141,544; 5,257,505; and 5,375,422.
Compander technology comprising an integrally geared design with one or more expansion stages and one or more compressor stages has previously been disclosed, for example, by Cryostar Industries. The expansion of gas allows for energy to be extracted or harnessed by the use of an expander device. The expander is coupled with a matching compressor, thereby creating a stage compression as is useful in the process. Auxiliary compression is often required to produce the total amount of compression requirements.